KR20150112508A - High strength cold-rolled steel sheet and method for manufacturing the same - Google Patents

Abstract

Disclosed are a high-strength cold-rolled steel sheet having excellent shape fixability and a low yield ratio, and a manufacturing method thereof. According to the present invention, the method to manufacture a high-strength cold-rolled steel sheet comprises the steps of: conducting, in an intercritical interval of austenite-ferrite, an annealing heat treatment of a steel sheet composed of 0.10-0.14 wt% of carbon (C), 0.25-0.35 wt% of silicon (Si), 2.2-2.4 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P), 0.005 wt% or less of sulfur (S), 0.02-0.06 wt% of aluminum (Al), 0.35-0.45 wt% of chromium (Cr), 0.06-0.09 wt% of molybdenum (Mo), 0.006 wt% or less of nitrogen (N), and the remainder consisting of iron (Fe) and inevitable impurities; primarily cooling the steel sheet having completed the annealing heat treatment to a temperature of 480-520°C; conducting an isothermal heat treatment of the primarily cooled plate; and secondarily cooling the steel sheet having completed the isothermal heat treatment to a martensite region.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength cold rolled steel sheet,

The present invention relates to a high strength cold rolled steel sheet manufacturing method, and more particularly, to a high strength cold rolled steel sheet having excellent shape-formability and a manufacturing method thereof.

Since most of the automobile parts are formed by press working, excellent press workability is required. Particularly, as the automobile design becomes complicated and the needs of consumers are diversified, a steel plate having high strength and excellent formability is required.

Therefore, high-strength steel plates using a dual phase are used for parts requiring high energy absorption performance in the event of a vehicle collision such as members, pillars, and bumper reinforcements.

The high-strength steel sheet for automobiles is roughly classified into hot rolling, cold rolling and annealing. The hot-rolled plate having ferrite and pearlite is cold-rolled to a final thickness of the product, And then cooled to produce a cold-rolled steel sheet.

At this time, the austenite formed during the annealing process is transformed into martensite or bainite by controlling the rate during the cooling process, and when it is transformed into martensite, it is called abnormal texture steel. As the fraction of martensite increases, the strength increases and the ductility increases as the ferrite ratio increases. Therefore, the ideal structure steel has been optimized to optimize the fraction of martensite and ferrite to ensure tensile strength and yield strength.

However, it is possible to secure proper martensite but high yield strength problem occurs. The high yield strength is difficult to obtain due to the spring back phenomenon during press working, and it is difficult to manufacture the bumper reinforcements, members, and pillar parts used in the high strength cold-rolled steel sheet.

As a prior art related to the present invention, there is a super high strength manufacturing method disclosed in Korean Patent Laid-Open Publication No. 10-2005-0063981 (published on June 29, 2005), which is excellent in bending workability.

An object of the present invention is to provide a high strength cold rolled steel sheet having a high tensile strength of 960 MPa or more and a low resistance ratio of 70% or less, and excellent in shape fixability, and a method of manufacturing the same.

In order to achieve the above object, the present invention provides a method for manufacturing a high strength cold rolled steel sheet, comprising: 0.10 to 0.14% carbon, 0.25 to 0.35% silicon, 2.2 to 0.35% manganese (Mn) (Al): 0.02 to 0.06%, Cr: 0.35 to 0.45%, Mo: 0.06 to 0.09%, phosphorus (P): 0.03% %, Nitrogen (N): 0.006% or less, and the balance of iron (Fe) and unavoidable impurities in an austenite-ferrite phase reversed zone; Firstly cooling the annealed heat treated steel sheet to 480 to 520 ° C; Subjecting the primarily cooled plate to a constant temperature heat treatment; And secondarily cooling the heat-treated steel sheet to the martensite region.

At this time, the annealing heat treatment is preferably performed at 780 to 820 ° C for 60 to 150 seconds.

Also, it is preferable that the primary cooling is performed at an average cooling rate of 10 to 50 DEG C / sec.

Between the constant-temperature heat treatment and the secondary cooling, a hot-dip galvanizing step may further be included.

In order to achieve the above object, a high strength cold rolled steel sheet according to an embodiment of the present invention comprises 0.10 to 0.14% of carbon (C), 0.25 to 0.35% of silicon (Si), 2.2 to 2.4% of manganese (Mn) , 0.03% or less phosphorus (P), 0.005% or less sulfur (S), 0.02-0.06% aluminum (Al), 0.35-0.45% chromium (Cr), 0.06-0.09% molybdenum 0.006% or less of nitrogen (N) and the balance of iron (Fe) and inevitable impurities, and has a composite structure including ferrite and martensite, and has a tensile strength of 960 MPa or more and a yield ratio of less than 70%.

At this time, the martensite fraction may be 38 to 40% in area ratio.

According to the method for manufacturing a high strength steel sheet according to the present invention, a tensile strength of 960 MPa or more is obtained through annealing in an austenite-ferrite anomalous zone through pearlite, bainite transformation delay or austenite-ferrite phase transition through an alloy component of carbon, manganese, chromium, molybdenum, A cold rolled steel sheet having a low resistance ratio of less than 70% can be produced.

Therefore, the high-strength cold-rolled steel sheet according to the present invention can be utilized for automobile parts having a complicated shape because of excellent shape-crystallizing property.

1 is a flowchart schematically showing a method of manufacturing a high strength cold rolled steel sheet according to an embodiment of the present invention.
Fig. 2 shows a microstructure photograph of the specimen 5. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method of manufacturing a high strength cold rolled steel sheet according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

High strength cold rolled steel sheet

The high strength cold rolled steel sheet according to the present invention contains 0.10 to 0.14% of carbon (C), 0.25 to 0.35% of silicon (Si), 2.2 to 2.4% of manganese (Mn), 0.03% or less of phosphorus 0.005% or less of sulfur (S), 0.02 to 0.06% of aluminum (Al), 0.35 to 0.45% of chromium (Cr), 0.06 to 0.09% of molybdenum (Mo) .

The rest of the alloy components are composed of iron (Fe) and unavoidable impurities.

The cold-rolled steel sheet according to the present invention may be a cold-rolled steel sheet in a pure meaning, or a hot-dip galvanized steel sheet or a galvannealed galvanized steel sheet on which a hot-dip galvanized layer is formed.

Hereinafter, the role and content of each component included in the high strength cold rolled steel sheet according to the present invention will be described.

Carbon [C]: 0.10 to 0.14 wt%

Carbon (C) is an important element that determines the fraction and hardness of martensite in a composite structure steel. When it is added in an amount of less than 0.10 wt%, it is not easy to secure strength. Also, when carbon was added in an amount exceeding 0.14 weight%, ductility and stretch-flangeability with the increase in strength were lowered, and the upper limit was limited to 0.14 weight%.

Silicon [Si]: 0.25-0.35 wt%

Silicon (Si) is a solid solution strengthening element which accelerates the refinement of steel and carbon enrichment in austenite and improves the flowability of molten metal during the welding among the steels to which Mn is added in order to minimize the residual inclusions in the weld, It improves the strength without inhibiting the balance of elongation and slows the diffusion rate of carbon in the ferrite, thereby suppressing carbide growth and stabilizing ferrite to improve elongation. The above effect is remarkable when the silicon content is 0.25% by weight or more. However, when the content of silicon exceeds 0.35% by weight, weldability and the like are deteriorated.

Manganese [Mn]: 2.2 to 2.4 wt%

Manganese (Mn) stabilizes austenite as a solid solution strengthening element to lower an ideal reverse annealing temperature, and martensite tends to be generated even at a low critical cooling rate. However, when the addition amount of manganese is less than 2.2 wt%, a small amount of manganese is transformed into bainite and pearlite in the annealing heat treatment section, which causes the strength to be lowered. On the other hand, when the added amount of manganese exceeds 2.4 wt%, manganese bands are developed at the center of the material thickness and the elongation rate is lowered.

Phosphorus [P]: not more than 0.03% by weight

Phosphorus (P) is an element that increases the strength of a steel sheet by solid solution strengthening. It is an effective element for inhibiting carbide formation, and plays a role of preventing elongation decrease due to carbide formation in the overpressure zone. It is also effective for obtaining martensite by improving manganese equivalence. The excess amount of Fe3P forms stagedite of Fe3P to cause hot brittleness. Therefore, the addition amount is limited to 0.03 wt% or less.

Sulfur [S]: 0.005 wt% or less

Sulfur (S) inhibits toughness weldability and increases MnS nonmetallic inclusions, inhibiting the incombustibility effect of Mn and causing processing cracks. In case of excessive addition, coarse inclusions are increased to deteriorate fatigue characteristics, so that the upper limit value is limited to 0.005% or less.

Aluminum [Al]: 0.02 to 0.06 wt%

The aluminum (Al) element is an element mainly used as a deoxidizing agent, which purifies the ferrite to improve the elongation and stabilizes the austenite by increasing the carbon concentration in the austenite. Also, it acts as a layer between the iron and the zinc plated layer and improves the plating property. It is an effective element for suppressing the formation of manganese band in the hot-rolled coil. In order to obtain the above-mentioned effect, the addition amount is preferably 0.02 wt% or more, but when it is added in excess of 0.06 wt%, the performance is deteriorated and AlN is formed in the slab to cause hot cracking, Respectively.

Cr [Cr]: 0.35-0.45 wt%

Chromium (Cr) is an austenite stabilizing element and has an entanglement effect. The effect is insufficient when added in a small amount less than 0.35% by weight, and the plating ability can be inhibited when it is added in excess of 0.45% by weight.

Molybdenum [Mo]: 0.06 to 0.09 wt%

Molybdenum (Mo) is an austenite stabilizing element and has an entanglement effect. When molybdenum is added at 0.06 wt% or more, the effect of increasing the strength is remarkable. However, even if the molybdenum content exceeds 0.09% by weight, the economical efficiency may be lowered without further improving the effect.

Nitrogen [N]: Not more than 0.006%

Nitrogen (N) is refined by the formation of AlN, but since it is supersaturated upon cooling in the galvannealing process of the galvanized layer during hot dip galvanizing, the uniform elongation is lowered and is limited to 0.006 wt% or less.

The high strength cold rolled steel sheet according to the present invention having the above alloy component has a composite structure including ferrite and martensite through a manufacturing process described later, and can exhibit a tensile strength of 960 MPa or more and a yield ratio of less than 70%.

In the high-strength cold-rolled steel sheet according to the present invention, the martensite fraction may be in the range of 38 to 40% by securing an austenite fraction of about 83% at a substantial area ratio during annealing.

Method of manufacturing high strength cold rolled steel sheet

1 is a flowchart schematically showing a method of manufacturing a high-strength steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, the method for manufacturing a high strength cold rolled steel sheet includes a annealing heat treatment step (S110), a primary cooling step (S120), a constant temperature heat treatment step (S130), and a secondary cooling step (S140). Between the constant-temperature heat treatment step (S130) and the second cooling step, a hot-dip galvanizing step (S135) may be further included, and after the hot-dip galvanizing, further alloying heat treatment may be performed.

First, in the annealing heat treatment step (S110), the cold-rolled cold-rolled steel sheet having the above-described alloy composition is heated and annealed. In the present invention, an annealing heat treatment is performed in the austenite-ferrite phase. It is possible to prevent grain boundary coarsening of the steel sheet through the reverse annealing process, and thereby it is possible to manufacture a cold rolled steel sheet having excellent strength and elongation.

The austenite phase fraction can be controlled through the annealing heat treatment. Through the first cooling (S120), the constant temperature annealing (S130) and the second cooling (S140) described below, the martensite fraction is 38 to 40% It is possible to manufacture a cold-rolled steel sheet having a ferrite-martensite structure or more.

The annealing heat treatment is more preferably performed at 780 to 820 캜 for 60 to 150 seconds. The austenite can be secured in an area ratio of about 83% in the temperature and time ranges, and the martensite can be formed in an area ratio of 38 to 40% in the final microstructure.

In the primary cooling step (S120), the annealed steel sheet is first cooled to 480 to 520 ° C.

The primary cooling may be a roll quenching method, a gas jet method, or the like.

Through the primary cooling, about 60% of the area ratio of ferrite is generated. At this time, in the present invention, the continuous cooling transformation curve (CCT Curve) can be shifted to the right according to the addition of carbon, manganese, chromium, and molybdenum, so that pearlite and bainite transformation can be delayed.

On the other hand, the primary cooling is preferably performed at an average cooling rate of 10 to 50 DEG C / sec. If the primary cooling is carried out at an average cooling rate of less than 10 캜 / sec, a large amount of pearlite, bainite transformation may occur. In addition, when the primary cooling exceeds 50 캜 / sec, sufficient ferrite transformation may not occur, and material deviation may occur due to rapid cooling.

Next, in the constant-temperature heat treatment step (S130), the primary-cooled plate is heat-treated at a constant temperature. The stretching ratio of the steel sheet can be improved through the constant temperature heat treatment. The quenching heat treatment can be carried out in such a manner that the primary cooled quartz plate is held for about 100 to 400 seconds near the primary quenching temperature.

After the constant-temperature heat treatment, the hot-dip galvanizing step (S135) may be further performed. In the present invention, the content of silicon, manganese, and chromium, which are elements capable of deteriorating the plating ability upon addition of a large amount, is limited to about 0.3 wt%, 2.3 wt%, and 0.4 wt%, respectively.

Next, in the second cooling step, the quenched or hot-dip galvanized steel sheet is secondarily cooled to the martensite region to form ferrite and martensite, and the martensite fraction forms a final microstructure having an area ratio of 38 to 40% do. The final cooling may be performed at an average cooling rate on the order of 10-100 [deg.] C / sec.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

(P): 0.01% or less; sulfur (S): 0.003% by weight; aluminum (Al) , Cold annealed specimens of 0.03% Cr, 0.4% Cr, 0.07% Mo, 0.004% N and the balance iron and unavoidable impurities were annealed for 100 seconds under the conditions shown in Table 1, Lt; RTI ID = 0.0 > 20 C / sec. ≪ / RTI > Thereafter, a constant-temperature heat treatment was performed for 200 seconds at the first cooling end temperature, followed by final cooling to 250 占 폚 at an average cooling rate of 50 占 폚 / sec and air cooling.

[Table 1]

2. Evaluation of mechanical properties

Tensile tests were conducted on the prepared specimens 1 to 9, and the results are shown in Table 1.

Referring to Table 1, all specimens 1 to 9 satisfying the annealing heat treatment temperature and the first cooling finishing temperature conditions shown in the present invention exhibited a tensile strength of 760 MPa or more and a yield ratio of less than 70%, and a martensite fraction or area ratio of 38 To 40%. ≪ / RTI >

Referring to Table 1, it can be seen that the yield ratio tends to decrease as the annealing temperature is lower, and it is preferable to conduct the annealing heat treatment at 780 to 800 ° C to obtain the resistance ratio of 65% or less.

As described above, such a low resistance ratio contributes to securing excellent shape crystallinity, and it is possible to reduce the weight of automobile parts and the like through high strength.

Fig. 2 shows a microstructure photograph of the specimen 5. Fig. Referring to FIG. 2, it can be seen that the manufactured specimen 5 has a composite structure of ferrite and martensite. As described above, in the case of the specimen 5, the measurement result of the martensite fraction was 38.3 vol%.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

Claims (6)

(P): 0.03% or less, sulfur (S): 0.005 wt% or less, and more preferably 0.1 to 0.1 wt% of carbon (C), 0.25 to 0.35 wt% (Fe) and inevitable impurities (Fe) are contained in an amount of not more than 0.1% by mass, aluminum (Al): 0.02 to 0.06%, chromium (Cr): 0.35 to 0.45%, molybdenum Annealing the steel sheet in an austenite-ferrite phase reversed zone;
Firstly cooling the annealed heat treated steel sheet to 480 to 520 ° C;
Subjecting the primarily cooled plate to a constant temperature heat treatment; And
And secondarily cooling the hot-heat-treated steel sheet to a martensite region.
The method according to claim 1,
Wherein the annealing heat treatment is performed at 780 to 820 캜 for 60 to 150 seconds.
The method according to claim 1,
Wherein the primary cooling is performed at an average cooling rate of 10 to 50 DEG C / sec.
The method according to claim 1,
Wherein the hot-dip galvanizing step further comprises a hot-dip galvanizing step between the constant-temperature heat treatment and the secondary cooling.
(P): 0.03% or less, sulfur (S): 0.005 wt% or less, and more preferably 0.1 to 0.1 wt% of carbon (C), 0.25 to 0.35 wt% (Fe) and inevitable impurities (Fe) are contained in an amount of not more than 0.1% by mass, aluminum (Al): 0.02 to 0.06%, chromium (Cr): 0.35 to 0.45%, molybdenum Lt; / RTI >
Ferrite and martensite, and has a tensile strength of 960 MPa or more and a yield ratio of less than 70%.
6. The method of claim 5,
Wherein the martensite fraction is 38 to 40% in area ratio.

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